The present invention relates to multichannel cochlear prosthesis, and more particularly to a multichannel cochlear prosthesis that offers flexible control of the stimulus waveforms.
Cochlear prostheses produce sensations of sound in deaf patients by direct electrical stimulation of the auditory nerve. In modern, multichannel cochlear prostheses, several different sites are stimulated at various distances along the cochlea to evoke the different pitches of sound perception that are normally encoded by nerve activity originating from the respective sites. The patterns of electrical stimulation are derived from acoustic signals picked up by a microphone and transformed by a so-called speech processor that is programmed to meet the particular requirements of each patient. Several different schemes for processing the acoustic signal and transforming it into electrical stimuli have been developed and are well-described in the scientific literature and various patents. All of these schemes can be divided into two basic types on the basis of the waveforms of the electrical stimuli:
i) Analog waveforms, which are essentially filtered versions of the continuous acoustic waveform, usually involving dynamic range compression, bandpass filtering and scaling to the stimulus current ranges that evoke a satisfactory range of auditory sensations from threshold of perception to maximal comfortable loudness. This produces a rich but poorly controlled set of resultant waveforms.
ii) Biphasic pulses, which consist of a single cycle of a square wave in which current flows in one direction at a specified magnitude and for a specified brief period of time and is followed immediately by an opposite direction of current of a similar magnitude and duration. These pulses are most often delivered in sequence to various sites, with the instantaneous magnitude at each site proportional to some measure of the amount of energy present in a particular frequency band of the acoustic waveform. The result is an impoverished but precisely controlled set of stimulus waveforms.
Both of these stimulus waveform types have been selected because they are relatively easy to produce and modulate electronically for real-time encoding of speech and because they guarantee a charge-balanced alternating current at the electrodes, avoiding net direct current that is known to cause electrolytic damage to both electrodes and body tissues.
Recent findings regarding the complex biophysical phenomena associated with the electrical excitation of neurons and psychophysical phenomena regarding the interpretation of neural activity by the auditory nervous system suggest that the quality and intelligibility of speech percepts evoked by a cochlear prosthesis may be improved in a given patient by more specific manipulations of the electrical stimulus waveforms tailored to that patient. In particular, more complex sequences of polarities, with or without pauses between phases and sites, and with or without simultaneous current delivery at more than one site, appear to be desirable. There is thus a need in the art for a cochlear stimulation system that allows complex stimulation waveforms to be individually tailored for each stimulation site.
International Publication WO 91/03913 (Published Mar. 21, 1991), corresponding to International Application PCT/AU90/00407, filed Sep. 7, 1990, describes a multichannel cochlear prostheses that includes a speech processor coupled to a cochlear stimulator, and wherein the speech processor includes stimulation parameters stored in a random access memory (RAM). These parameters, as stored in RAM, are referred to as a MAP. An audiologist "generates" and fine tunes each patient's MAP using a diagnostic and programming system (DPS) The DPS is used to administer appropriate tests, present controlled stimuli, and confirm and record test results. Unfortunately, however, the controlled stimuli that may be administered during such tests is restricted, being generally limited to balanced biphasic pulses (i.e., biphasic pulses as shown in FIG. 5 or 6), having equal negative and positive phases) having variations in pulse amplitude, pulse width, and pulse rate.
European Patent Application 0 247 649 A1 likewise shows a multichannel cochlear prostheses that is very similar to that which is disclosed in the above-referenced International Publication WO 91/03913. However, insofar as can be determined from that which is disclosed, the controlled stimuli that may be administered by a suitable diagnostic and programming system (DPS), suffers from the same restrictions, i.e., only a limited number of parameters may be varied during such tests. The '649 European Patent Application thus fails to show or suggest that complex stimulation patterns can or should be individually tailored for each stimulation site.
U.S. Pat. No. 5,073,939 discloses a very structured dynamic time warping (DTW) apparatus for use in a speech processing system wherein stored template data is compared to uttered speech to find a best match.
U.S. Pat. No. 4,918,745 shows a multichannel cochlear implant system that employs a very precise time multiplexing scheme used between multiple channels through the use of analog switches. With this scheme, only the electrode coupled to the correct analog switch receives the audio signal for the appropriate channel at the appropriate time.
International Publication WO 95/01709 (published Jan. 12, 1995), corresponding to International Application No. PCT/AU94/00370, filed Jul. 1, 1994, shows a cochlear implant device wherein the stimulation is dictated by a predefined instruction set. The predefined instruction set includes only limited variations, such as the selection of different modes of stimulation and/or use of different electrode geometries.